This invention generally relates to sensors. More particularly, it relates to low power differential sensors. Even more particularly, it relates to a device for low power sensing and transmitting data.
Smart sensors are being developed for use in roads, bridges, dams, buildings, towers, and vehicles. The sensors may provide many types of information, including displacement, strain, speed, acceleration, temperature, pressure, and force. For remote sensing one challenge has been to provide sensors that consume very low power for reading the sensor and transmitting the data.
Available sensors have required continuous energizing either for operation or for data transmission, and have required substantial power supplies. For example, a paper, xe2x80x9cMultichannel Strain Gauge Telemetry for Orthopaedic Implants,xe2x80x9d by G. Bergmann, et al., J. Biomechanics Vol. 21 no. 2 pp 169-176, 1988, describes remote powering of a Wheatstone bridge with active strain gauges that require continuous power. A paper, xe2x80x9cRemotely powered, multichannel, microprocessor based telemetry systems for smart implantable devices and smart structures, by Chrisopher Townsend, et al, describes an implantable sensor telemetry system that uses low power microprocessors integrated circuits, Wheatstone bridge signal conditioning, and a remote powering system. The Wheatstone bridge has advantage in providing temperature compensation. However, the bridge circuit also requires a continuous voltage and flow of current, so substantial energy is eventually used. Conventional Wheatstone bridge signal conditioners, such as Townsend""s, require instrumentation amplifiers and analog to digital converters which increase the power demand, size, and complexity of these systems.
International patent WO 87/00951 shows an inductive sensor used as the feedback element in an astable multivibrator. This circuit requires a non-differential sensor, which results in poor temperature stability. In addition, the astable multivibrator requires continuous power to operate.
A book, xe2x80x9cCapacitive sensors design and Applications,xe2x80x9d by L. K. Baxter, IEEE Press, 1997, shows a microcontroller providing a train of pulses or a microcontroller providing a single interrogation pulse to excite a capacitive limit switch. However, the circuit described by Baxter does not provide a way to measure more than the two positions of the capacitor and does not compensate for changes in temperature.
A paper, xe2x80x9cMicrominiature, high resoluton, linear displacement sensor for peak strain detection in smart structures,xe2x80x9d by Steven W. Arms, et al., proceedings of the SPIE 5th Annual International Conference on Smart Structures and Materials, San Diego, Calif., Mar. 1-5, 1998, describes a differential method of capturing the peak displacement of a member attached to a structure without requiring any power. The paper did not describe micropower methods for remote interrogation.
Thus, a better system for acquiring and transmitting data is needed that uses less energy and that provides temperature compensation, and this solution is provided by the following invention.
It is therefore an object of the present invention to provide a circuit for collecting sense data that avoids a continuous flow of current and use of power;
It is a further object of the present invention to lower power requirements for a sensor by providing a circuit in which a single signal, such as a single pulse, is sufficient for performing a measurement;
It is a further object of the present invention to combine a low power circuit for reading a sensor with a remotely powered interrogation system;
It is a further object of the present invention to provide a differential sensor in a Wheatstone bridge configuration with a pulse signal to provide a low power data sensing circuit;
It is a feature of the present invention that the Wheatstone bridge provides for a temperature compensated reading of the differential sensor;
It is a further feature of the present invention that the remotely powered interrogation system provides power for running the sensor; and
It is an advantage of the present invention that the circuit for reading a sensor uses very low power.
These and other objects, features, and advantages of the invention are accomplished by a electronic circuit comprising a first electrode, a second electrode, a third electrode and a fourth electrode. The circuit also includes a differential sensor comprising a first variable element connected to a second variable element at the first electrode. The first variable element is also connected to the second electrode. The second variable element is also connected to the third electrode. A fixed device is connected between the second electrode and the fourth electrode. A source of a stimulation is connected to apply a stimulation across the first and the fourth electrodes. A timing sensitive circuit is configured to measure timing of a signal appearing at the second electrode that arises from the stimulation applied across the first and fourth electrodes.
Another aspect of the invention is accomplished by a method of reading a sensor comprising several steps. The first step is providing a differential sensor having a first variable element and a second variable element. Next, providing a comparator. Then providing a signal to the first variable element wherein the sensor produces an output depending on magnitude of the first variable element. Finally, using the comparator for providing a signal that is a measure of that magnitude.
Another aspect of the invention is accomplished by a method of using an electronic circuit, comprising the step of providing a circuit comprising a sensor, a circuit for reading the sensor, and a circuit for transmitting data. The next step is wirelessly providing power to the circuit from a remote source of power. Then, sensing a change in an environmental condition with the sensor. Then, reading the sensor with the circuit for reading the sensor, wherein only a single stimulation signal to the sensor is needed to read the sensor. Then, providing the reading to a transmitting circuit and transmitting the data with the transmitting circuit.